prevector: Suppress two -Wmaybe-uninitialized (gcc) warnings in indirect_ptr(...) #13902

Suppress two -Wmaybe-uninitialized warnings in indirect_ptr(...):

[…]
./prevector.h: In function ‘bool IsWitnessStandard(const CTransaction&, const CCoinsViewCache&)’:
./prevector.h:162:79: warning: ‘prevScript.prevector<28, unsigned char>::_union.prevector<28, unsigned char>::direct_or_indirect::<anonymous>.prevector<28, unsigned char>
     ::direct_or_indirect::<unnamed struct>::indirect’ may be used uninitialized in this 
     function [-Wmaybe-uninitialized]

     T* indirect_ptr(difference_type pos) { return reinterpret_cast<T*>(_union.indirect) + pos; }
                                                                        ~~~~~~~^~~~~~~~

[…]
In file included from ./hash.h:11:0,
                 from ./pubkey.h:10,
                 from ./key.h:10,
                 from test/script_p2sh_tests.cpp:7:
./prevector.h: In member function ‘void script_p2sh_tests::sign::test_method()’:
./prevector.h:162:79: warning: ‘sigSave.prevector<28, unsigned char>::_union.prevector<28, unsigned char>::direct_or_indirect::<anonymous>.prevector<28, unsigned char>::direct_or_indirect::<unnamed struct>::indirect’ may be used uninitialized in this function [-Wmaybe-uninitialized]
     T* indirect_ptr(difference_type pos) { return reinterpret_cast<T*>(_union.indirect) + pos; }
                                                                        ~~~~~~~^~~~~~~~
./prevector.h:181:69: warning: ‘sigSave.prevector<28, unsigned char>::_union.prevector<28, unsigned char>::direct_or_indirect::<anonymous>.prevector<28, unsigned char>::direct_or_indirect::<unnamed struct>::indirect’ may be used uninitialized in this function [-Wmaybe-uninitialized]
                 _union.indirect = static_cast<char*>(realloc(_union.indirect, ((size_t)sizeof(T)) * new_capacity));
                                                              ~~~~~~~^~~~~~~~
[…]

Related change from @TheBlueMatt in c73b8be2440e424f304fda22e43240e8b90f9b00.

Ping @sipa who wrote the original prevector code :-)

What version of gcc are you using? I think this might be related to https://gcc.gnu.org/bugzilla/show_bug.cgi?id=36750 which was fixed in GCC 5.

@Empact I'm using vanilla gcc 7.3.0 under Ubuntu 18.04.1 LTS:

$ g++ --version
g++ (Ubuntu 7.3.0-16ubuntu3) 7.3.0
Copyright (C) 2017 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

$ cat /etc/lsb-release
DISTRIB_ID=Ubuntu
DISTRIB_RELEASE=18.04
DISTRIB_CODENAME=bionic
DISTRIB_DESCRIPTION="Ubuntu 18.04.1 LTS"
$ dpkg -s g++ | grep -E Ver\|Dep
Version: 4:7.3.0-3ubuntu2
Depends: cpp (>= 4:7.3.0-3ubuntu2), gcc (>= 4:7.3.0-3ubuntu2), g++-7 (>= 7.3.0-12~), gcc-7 (>= 7.3.0-12~)

Nothing fancy – as vanilla as it gets :-)

I strongly dislike this solution, if this warning points to a real issue, that issue should be solved instead but let's not riddle the source code with compiler-specific pragmas

@laanwj Is it a real issue? I thought it was a false positive

I'm not a fan either, better to fix, if an issue exists, and take it up with the compiler if it's a false positive. Does the issue exist with GCC 8.2?

Possible cause: only the default constructor explicitly initializes _union

@practicalswift maybe see if #14028 fixes the warning?

@Empact The fix in #14028 fixes the warning. Thanks!

Closing this in favor of #14028. Thanks @Empact!